This invention relates to the treatment of fibers for use in the resin transfer/injection molding of high density continuous fiber preforms.
Manufacturing processes for the fabrication of high quality composite, thick walled (i.e., greater than about 0.25-inch) internal or external pressure vessels or other filament wound articles, comprise the winding of wet or prepreg fibers onto a forming mandrel. Filament winding processes, whether wet or prepreg, generally produce high void content structures (about 4 to 10% voids). Structures produced by these processes also exhibit fiber waviness due to resin flow between internal plies.
Filament wound structures can be cured in autoclaves to eliminate voids, but such curing increases the cost of the structures. Autoclave curing also increases internal fiber waviness due to resin flow.
Voids and fiber waviness can be minimized, if not eliminated, by the method of multiple cures of thin layers. Although successful, this method greatly increases the cost of the product. Multiple cures also lead to problems at the interface(s) between such multiple layers.
Processes that produce composites having 2 to 4% voids generally exhibit about 30% reduction in composite compressive strength. The stress concentration in composites having only a 2.5.degree. misalignment causes roughly 50% strength reduction, as compared to composites which are free of voids and fiber waviness.
Resin transfer/injection molding processes have long been acknowledged as producing low cost, dimensionally accurate components at moderate production quantities (about 500 to 5,000 units). Resin transfer molding is a closed mold, low pressure process applicable to the fabrication of complex, high performance composite articles of both large and small size. Several different resin transfer molding processes are well known to those skilled in the art. The process is differentiated from various other molding processes in that reinforcement material, such as fiberglass or other fiber reinforcement material, is placed separately into a molding tool cavity and then combined with resin within the mold cavity to form a fiber reinforced plastic ("FRP") composite product. Typically, a pre-shaped fiber reinforcement (preform) is positioned within a molding tool cavity and the molding tool is then closed. A feed line connects the closed molding tool cavity with a supply of liquid resin and the resin is pumped or otherwise transferred into the tool cavity where it impregnates and envelops the fiber reinforcement, and subsequently cures. The cured or semi-cured FRP product is then removed from the molding tool cavity. As used herein, the terms resin transfer molding and "RTM" refer to any molding process wherein a fiber reinforcement material or structure is positioned within a mold into which resin is subsequently introduced.
The key to the production of high performance components having low voids, low fiber waviness and high fiber content lies in the fabrication of high density, i.e., tightly packed, continuous fiber preforms. Such fabrication requires high fiber tension to compact the dry fiber preform. High fiber tension can, however, seriously degrade, by fiber damage and breakage, the strength of dry reinforcing fibers such as carbon, glass or other materials. Further, the compaction of dry fibers as successive layers are applied causes additional damage due to abrasion.
It is therefore an object of this invention to provide a method for treating fibers for use in the resin transfer/injection molding of high density continuous fiber preforms.
It is another object of this invention to provide a composition for treating fibers for use in the resin transfer/injection molding of high density continuous fiber preforms.